Polymers obtained by ring-opening polymerization of cycloolefin monomers that contain the norbornene moiety, referred to herein as cycloolefins or cycloolefin monomers, are well known. For example, U.S. Pat. Nos. 4,136,249; 4,178,424; 4,136,247; and 4,136,248, assigned to the same assignee as the present invention, describe such polymers and each is incorporated herein by reference for the description of the polymers therein.
Depending on the specific cyloolefins chosen, ring-opening polymerization of cycloolefins yields unsaturated linear, branched and crosslinked polymers. These polymers are known to exhibit attractive property profiles for many polymer applications, such as automotive and non-automotive body panel equipment housings, furniture, window frames and shipment dunnage.
Dicyclopentadiene, for example, is a common cycloolefin monomer used to prepare ring-opened polymerized polymers in that this cycloolefin monomer is readily available as a by-product in ethylene production. U.S. Patents directed to polymers from dicyclopentadiene include U.S. Pat. Nos. 3,778,420; 3,781,257; 3,790,545; 3,853,830; 4,002,815; and 4,239,874. Other well known cycloolefin monomers include bicyclic norbornene and substituted bicyclic norbornenes, as do other U.S. patents such as U.S. Pat. Nos. 3,546,183; 2,721,189; 2,831,037; 2,932,630; 3,330,815; 3,367,924; 3,467,633; 3,836,593, 3,879,343; and 4,010,021. The above patents are incorporated herein by reference for their disclosure of polymers obtained from ring-opening polymerization of bicyclic norbornenes and substituted bicyclic norbornenes. Tetracyclododecene and substituted tetracyclododecenes are also well known cycloolefins. These are made by Diels-Alder reaction of cyclopentadiene with bicyclic norbornene or the appropriate substituted bicyclic norbornene. Ring-opening polymerization of tetracyclododecene with other bicyclic olefin comonomers has been disclosed in U.S. Pat. No. 3,557,072, incorporated herein by reference for the polymerizations disclosed therein.
Work has been done on bulk ring-opening polymerization of cycloolefins. Bulk polymerization is defined as polymerization in the absence of a solvent or diluent for the feed monomer. Minchak, U.S. Pat. No. 4,426,502, discloses a bulk polymerization process for "norbornene type monomers" which include norbornene, dicyclopentadiene, tricyclopentadiene (cyclopentadiene trimer) and tetracyclododecene.
Early attempts at the bulk polymerization of cycloolefins were too rapid in the absence of a solvent and therefore, uncontrollable. Furthermore, initial bulk polymerization attempts resulted in materials that were very dark, had poor physical properties and poor appearance.
Further developments in the bulk polymerization of cycloolefins led to another approach, which, likewise, was unsuccessful. This approach was characterized by splitting a monomer charge into two equal portions, one containing a catalyst and the other containing a cocatalyst. The object was to mix the two portions of the monomer charge at room temperature and then transfer the mix to a heated mold where polymerization and hardening would occur very quickly. It was discovered that instantaneous reaction took place upon contact of the two portions, whereby a solid polymer barrier was formed between the two portions of the monomer charge, encapsulating some of the monomer from each portion which prevented mixing.
Minchak, in U.S. Pat. No. 4,425,502, describes a modified metathesis catalyst system for bulk ring-opening polymerization of cyclic olefins containing a norbornene group. The process includes the steps of mixing the monomer with an organoammonium molybdate or tungstate metathesis catalyst and an alkoxyalkyl aluminum halide cocatalyst at a temperature at which polymerization of the monomer remains essentially dormant for at least one hour, and conveying the resulting mixture to a mold maintained at a temperature at which polymerization of the monomer takes place in less than two minutes. The Minchak metathesis catalyst system allows for control of the instantaneous reaction between the two portions of the monomer charge, one containing metathesis catalyst and other containing cocatalyst, which permits adequate mixing of the two portions without encapsulation, thereby permitting use within a reaction injection molding (RIM) process.
In typical RIM operations, chargesof monomer are separately mixed with the two-parts of the metathesis-catalyst system to form feed solutions for charging a mixing head of a RIM machine. Once mixed, the monomer solutions with catalyst and cocatalyst are injected into a mold.
While the metathesis catalyst systems of Minchak provide adequate control over the bulk polymerization reaction to obtain molded structurally sound articles by RIM processes, there are other obstacles the manufacturers of molded articles must face. For example, if such a manufacturer is to take advantage of a bulk polymerization reaction of cycloolefins in a reaction injection molding process, the molder must obtain complete fill of the mold with the reactive formulation. Where appearance parts are to be molded the manufacturer is concerned with sink marks in the surface of the molded article. Molded products obtained from RIM processes are vulnerable to such defects. The use of blowing agents in the RIM formulation has been found to counteract this problem.
In RIM formulations, blowing agents have been found to (1) counteract the tendency of the formulation to shrink upon polymerization, i.e., prevent sink marks in the product surface and (2) prevent humid air from being drawn into the mold cavity. Humid air deactivates the metathesis catalyst system creating "wet surfaces" on the parts.
Newburg, in U.S. Pat. No. 4,535,097, describes a metathesis-catalyst system which incorporates a blowing agent when polymerizing dicyclopentadiene monomer. Other metathesis-catalyst systems incorporating blowing agents are disclosed in U.S. Pat. Nos. 4,458,037; 4,496,668; 4,496,669; 4,568,660; 4,584,425; 4,598,102; 4,604,408; 4,696,985; 4,699,963; and 4,703,068. These systems incorporate any conventional blowing agents used in RIM processes or related processes that do not poison or otherwise adversely affect the metathesis catalyst. Such catalyst systems may include blowing agents such as low boiling organic compound or an inert gas.
When blowing agents such as nitrogen, carbon dioxide, chlorofluorocarbons, methylene chloride and various low boiling hydrocarbons such as butane, pentane, hexane and heptane are added to the RIM formulation, the RIM process requires the addition of a surfactant to stabilize gases produced as microbubbles. This surfactant can interfere with the adhesion of fillers and reinforcements to the polymer matrix.
The present invention solves this problem by utilizing a microencapsulated blowing agent (microsphere) in the reaction injection molding formulation which does not require the addition of surfactant. Microspheres which encapsulate liquid blowing agents are well known.
Morehouse et al., in U.S. Pat. No. 3,615,972, disclose thermoplastic microspheres which encapsulate a liquid blowing agent. Such microspheres are said to be readily prepared from a wide variety of materials.
Garner, in U.S. Pat. No. 4,075,138, discloses a method for the preparation of a synthetic resinous thermoplastic microsphere employing 60-90 parts by weight vinylidene chloride and from 40-10 parts by weight of acrylonitrile.
One such microsphere is currently available under the trade name EXPANCEL.RTM.. EXPANCEL is a white, spherically formed particle with a shell consisting basically of a copolymer of vinylidene chloride and acrylonitrile. The polymeric shell encapsulates the blowing agent, liquid isobutane, under pressure.
These microspheres are known to expand when they are subjected to heat which softens the thermoplastic shell and simultaneously volatilizes the encapsulated hydrocarbon. Their ability to expand at a given temperature, along with their extremely low weight and their elasticity, have made them useful in a wide area of applications: (1) in printing ink to create a three-dimensional pattern on wallpaper and other textiles; (2) in paper and board and other fibre products to lower density and to improve bending stiffness; (3) in plastic products to lower weight and to improve impact resistance; (4) in paints and putties to improve applicability and to reduce weight; (5) in cables to improve capacity; (6) in explosives to improve sensitivity; (7) in synthetic foams; and (8) as an alternative to conventional blowing agents for some resins, as disclosed in Japanese Patent Publication JP-60-244511.
Microencapsulated blowing agents have been used in a reaction injection molding process as described in Japanese Patent Publication No. 59-98564 [Japanese Patent Publication (KOKAI) 60-244511]. A heat-expandable microcapsule is activated during the curing stage of a reaction injection molding process.
The authors of the Japanese Patent Publication disclose the use of a low boiling hydrocarbon liquid in an outer shell of vinylidene chloride and acrylonitrile encapsulant in manufacturing "polyurethane elastomer" molded products by reaction injection molding. It is said that the products may be made with no sink marks. The claimed dosage of thermally expanded microencapsulate is 0.001-20 parts by weight per 100 parts by weight reactive mixture.
While the authors of the Japanese Patent Publication do not "deny" the applicability of the invention to other polyurethane resin systems or other synthetic resins, cycloolefin monomers are not said to be particularly effective. Furthermore, there is no teaching of the use of microen-capsulated blowing agents in a reaction injection molding process which provide polymers of cycloolefinic monomers obtained by ring-opening polymerization.
The present invention is based on the discovery that adding microspheres to bulk polymerization formulations of cycloolefins provides the advantages of a blowing agent without the need for a surfactant.